1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and a method for fabricating the same. More specifically, the present invention relates to an LCD device including liquid crystal droplets isolated from one another by polymer walls, operated in a twisted nematic (TN) mode, a super-twisted nematic (STN) mode, an electrically controlled birefringence (ECB) mode, a ferroelectric liquid crystal (FLC) mode, a light scattering mode, and the like, and a method for fabricating the same.
2. Description of the Related Art
Various display modes have been employed for LCD devices presently available. For LCD devices utilizing electro-optical effects, for example, the TN mode and the STN mode using nematic liquid crystal have been realized. In recent years, LCD devices using FLC have reached the level of practical use.
A new mode of LCD device where the birefringence of liquid crystal is used to electrically control the transparent or white opaque state of liquid crystal has been proposed. In this mode, the ordinary refractive index of liquid crystal (LC) molecules and the refractive index of a polymer as a display medium are matched to each other, in principle. The transparent state is displayed when a voltage is applied to liquid crystal, aligning the liquid crystal in the direction of the electric field. The white opaque state, i.e., a light scattering state, is displayed when a voltage is not applied to liquid crystal, causing disorderly orientation of the liquid crystal. The following five fabrication methods have been proposed for this mode of LCD devices:
(1) A method where liquid crystal is encapsulated in a polymer capsule to provide a display medium (Japanese National Publication No. 58-501631) PA1 (2) A method where photocurable or thermosetting resin and liquid crystal are mixed together. The liquid crystal is separated from the resin by curing the resin to form a liquid crystal region in the resin (Japanese National Publication No. 61-502128) PA1 (3) A method where the diameter of a droplet-like liquid crystal region is controlled (Japanese Laid-Open Patent Publication No. 3-72317) PA1 (4) A method where liquid crystal is put in holes of a porous polymer film (Japanese Laid-Open Patent Publication No. 3-59515) PA1 (5) A method where polymer beads as a light scattering source are floated in liquid crystal confined between two transparent-electrodes facing each other. (Japanese Laid-Open Patent Publication No. 3-46621) PA1 forming strip-shaped transparent-electrodes on two substrates at least one of which is transparent, the transparent-electrodes not easily transmitting light within a certain wavelength range; PA1 attaching the two substrates to face each other such that the transparent-electrodes on the two substrates intersect each other; PA1 injecting a mixture of at least liquid crystal material and photocurable resin into a space between the attached two substrates; and PA1 irradiating the mixture with light from at least one side of the attached two substrates. PA1 forming first transparent-electrodes constituting pixel electrodes in a matrix on a first substrate of two substrates at least a second substrate of which is transparent, and forming second transparent-electrodes not easily transmitting light within a certain wavelength range, the second transparent-electrodes including thickened portions of a pattern corresponding to and facing the pixel electrodes and thinned portions; PA1 attaching the two substrates to face each other such that the first transparent-electrodes and the thickened portions intersect each other; PA1 injecting a mixture of at least liquid crystal material and photocurable resin into a space between the attached two substrates; and PA1 irradiating the mixture with light from the side of the second substrate.
However, the above methods have respective disadvantages as follows.
In method (1), the encapsulated liquid crystal constitutes an independent liquid crystal (LC) droplet. Accordingly, a driving voltage for changing the orientation of liquid crystal molecules is required for each LC droplet, resulting in increasing the driving voltage required for operating all such LC droplets simultaneously as a whole. This limits the applications of the resultant LCD device.
Methods (2) and (3) utilize phase separation. Accordingly, it is difficult to locate the LC droplets at precise two-dimensional positions in method (2), and it is difficult to control the diameter of the LC droplets precisely in method (3).
Since method (4) does not utilize phase separation at the formation of LC droplets, it is advantageous in that the range of applicable resin materials and liquid crystal materials is significantly large, and that the porous polymer film can be sufficiently purified. At present, however, this method has disadvantages as follows: The diameter of the LC droplets can not be satisfactorily controlled; and the positions of the LC droplets along a substrate surface cannot be precisely controlled.
Method (5) provides great light scattering capability. However, this method has a disadvantage as follows: It is difficult to uniformly disperse beads and thus obtain the same degree of scattering for all pixels, resulting in a nonuniform display.
As described above, in the LCD devices using polymer dispersed liquid crystals where LC droplets are dispersed in a polymer matrix, the shape of the LC droplets is not uniform, and the precise arrangement of the LC droplets along the substrate surface is difficult to achieve due to the limitation in the fabrication methods. Because of this failure to precisely arrange the LC droplets, different driving voltages are required for the respective LC droplets. This results in losing sharpness at the threshold in the electro-optic characteristic and relatively increasing the driving voltage. Further, since a number of LC droplets which scatter less light are present, the contrast of the resultant LCD device is relatively low. Moreover, since the shape of the LC droplets is not uniform and the precise arrangement of the LC droplets along the substrate surface is difficult to obtain, as described above, a large screen with high precision is not possible. In the case where the LCD device is driven by a duty driving method using an average value obtained by turning on and off a signal, increasing the duty ratio is not possible.
In order to overcome the above problems, Japanese Patent Application No. 5-30996 assigned to the assignee of this application proposed an LCD device 400 as shown in FIGS. 12 and 13. Referring to FIG. 12, a mixture 113 of liquid crystal material, photocurable resin, and a photopolymerization initiator is injected into a space between a pair of substrates 101a and 101b facing each other. A photomask 114 having light shading portions 110 and light transmitting portions 111 is put over the substrate 101a so that the light shading portions 110 cover pixel portions. Then, ultraviolet (UV) light 108 is emitted from the side of the photomask 114 to irradiate the mixture 113. By this irradiation, as shown in FIG. 13, the LC molecules aggregate in the pixel portions corresponding to weak light irradiation regions to form liquid crystal (LC) regions 106 in a display medium layer, while the polymers aggregates in the portions other than the pixel portions corresponding to strong light irradiation regions to form polymer walls 107 in the display medium layer. In this proposed LCD device 400, since the pixel portions are shaded from light by the photomask, it is possible to form the LC regions 106 in the pixel portions.
However, the LCD device 400 having the LC regions in the pixel portions still has the following problem: At the time of light irradiation, an optical pass difference arises between the photomask and the polymer wall formation portions by the thickness of the substrate 101a. In other words, unless the light incident to the substrate 101a is parallel, the resultant polymer walls may differ from the intended shape thereof. The polymer walls may be wider than the pattern of the photomask by light scattering, or the polymer and the liquid crystal may not be distinctly phase-separated. This lowers the display quality of the resultant LCD device.